The article "Revenue opportunities for GSM network operators" by G. Buckman, on pages 39 to 42 of the magazine "Mobile Communications International", issue 23, June 1995, (obtainable from MCI Sales, 3-6 Kenrick Place, London W1H 3FF Great Britain) describes a mobile-radio network that contains virtual private networks for mobile subscribers. The described mobile-radio network is structured according to the GSM (Global System for Mobile Communications) standard, and contains at least one base station for servicing a radio cell and a device for establishing and releasing calls between two mobile subscribers and a fixed network subscriber (see the figure on page 40 therein). So-called virtual private networks can be installed in the mobile-radio network described therein, by forming groups in which different mobile subscribers are logically assigned to each other. Each private network comprising such a group is also termed in the above article as a virtual private branch exchange having its own call directory. Mobile subscribers can access the private branch exchange by dialing a four-digit number and make cost-effective calls within their group, meaning within their virtual private network. The above article does not describe how such a virtual private network can be arranged for mobile subscribers within the mobile-radio network.
U.S. Pat. No. 5,345,502 describes the installation of virtual private networks within a mobile-radio network. In that case a virtual private network is constructed with the aid of an intelligent network (see figure therein). The intelligent network contains so-called service control points INSCP1 and INSCP2, which are connected to exchanges PISDN1 or PISDN2, meaning elements which must be additionally installed in the existing mobile-radio network, thus requiring the use of additional hardware and software.